I'm assuming that you meant magni, as the modi is a DAC having not used a vali yet i can not give you a comparison but i enjoyed the sound from my magni very much and found it dead quiet. note that there is a minor channel imbalance in the magni at low volumes.
(1) There are no stupid questions only stupid answers. (2) I try to turn my equipment off when i'm not using it. (like when you sleeping or at work.) (3) yes the magni has a channel imbalance at low volume that's perfectly normal.
Any musical usb interface should do, what i would do is find a local music store and go ask them about usb interfaces and see what they recommend. you can probably audition it there as well, so you know what your getting before hand.